Not applicable.
This invention is in the field of rapid prototyping, and is more specifically directed to the fabrication of three-dimensional objects by selective laser sintering.
The field of rapid prototyping of parts has, in recent years, made significant improvements in providing high strength, high density, parts for use in the design and pilot production of many useful articles. xe2x80x9cRapid prototypingxe2x80x9d generally refers to the manufacture of articles directly from computer-aided-design (CAD) data bases in an automated fashion, rather than by conventional machining of prototype articles according to engineering drawings. As a result, the time required to produce prototype parts from engineering designs has been reduced from several weeks to a matter of a few hours.
By way of background, an example of a rapid prototyping technology is the selective laser sintering process practiced in systems available from 3D Systems, Inc. of Valencia, Calif., in which articles are produced from a laser-fusible powder in layerwise fashion. According to this process, a thin layer of powder is dispensed and then fused, melted, or sintered, by laser energy that is directed to those portions of the powder corresponding to a cross-section of the article. Conventional selective laser sintering systems, such as the SINTERSTATION 2500plus system available from 3D Systems, Inc., position the laser beam by way of galvanometer-driven mirrors that deflect the laser beam. The deflection of the laser beam is controlled, in combination with modulation of the laser itself, to direct laser energy to those locations of the fusible powder layer corresponding to the cross-section of the article to be formed in that layer. The laser may be scanned across the powder in raster fashion, with modulation of the laser effected in combination therewith, or the laser may be directed in vector fashion. In some applications, cross-sections of articles are formed in a powder layer by fusing powder along the outline of the cross-section in vector fashion either before or after a raster scan that xe2x80x9cfillsxe2x80x9d the area within the vector-drawn outline. In any case, after the selective fusing of powder in a given layer, an additional layer of powder is then dispensed, and the process repeated, with fused portions of later layers fusing to fused portions of previous layers (as appropriate for the article), until the article is complete.
Detailed description of the selective laser sintering technology may be found in U.S. Pat. Nos. 4,863,538, 5,132,143, and 4,944,817, all assigned to Board of Regents, The University of Texas System, and in U.S. Pat. No. 4,247,508 assigned to 3D Systems, Inc., all incorporated herein by this reference. Laser power control systems for selective laser sintering systems are described in U.S. Pat. No. 6,085,122, issued Jul. 4, 2000, and in U.S. Pat. No. 6,151,345, issued Nov. 21, 2000, both assigned to 3D Systems, Inc., and also incorporated herein by reference. By way of further background, U.S. Pat. No. 5,352,405, issued Oct. 4, 1994 assigned to 3D Systems, Inc., and incorporated herein by reference, describes a method of scanning the laser across the powder in a selective laser sintering apparatus to provide a uniform time-to-return of the laser for adjacent scans of the same region of powder, thus providing uniform thermal conditions over the cross-section of each of multiple parts within the same build cylinder.
The selective laser sintering technology has enabled the direct manufacture of three-dimensional articles of high resolution and dimensional accuracy from a variety of materials including polystyrene, NYLON, other plastics, and composite materials such as polymer coated metals and ceramics. Polystyrene parts may be used in the generation of tooling by way of the well-known xe2x80x9clost waxxe2x80x9d process. In addition, selective laser sintering may be used for the direct fabrication of molds from a CAD database representation of the object to be molded in the fabricated molds; in this case, computer operations will xe2x80x9cinvertxe2x80x9d the CAD database representation of the object to be formed, to directly form the negative molds from the powder.
FIG. 1 illustrates, by way of background, the construction and operation of a conventional selective laser sintering system 100. As shown in FIG. 1, selective laser sintering system 100 includes a chamber 102 (the front doors and top of chamber 102 not shown in FIG. 1, for purposes of clarity). Chamber 102 maintains the appropriate temperature and atmospheric composition (typically an inert atmosphere such as nitrogen) for the fabrication of the article.
The powder delivery system in system 100 includes feed piston 114, controlled by motor 116 to move upwardly and lift a volume of powder into chamber 102. Two powder pistons 114 may be provided on either side of part piston 106, for purposes of efficient and flexible powder delivery, as used in the SINTERSTATION 2500 plus system available from 3D Systems, Inc. Part piston 106 is controlled by motor 108 to move downwardly below the floor of chamber 102 by a small amount, for example 0.125 mm, to define the thickness of each layer of powder to be processed. Roller 118 is a counter-rotating roller that translates powder from feed piston 114 to target surface 104. Target surface 104, for purposes of the description herein, refers to the top surface of heat-fusible powder (including portions previously sintered, if present) disposed above part piston 106; the sintered and unsintered powder disposed on part piston 106 will be referred to herein as part bed 107. Another known powder delivery system feeds powder from above part piston 106, in front of a delivery apparatus such as a roller or scraper.
In conventional selective laser sintering system 100 of FIG. 1, a laser beam is generated by laser 110, and aimed at target surface 104 by way of scanning system 142, generally including galvanometer-driven mirrors that deflect the laser beam. The deflection of the laser beam is controlled in combination with modulation of laser 110 itself, to direct laser energy to those locations of the fusible powder layer corresponding to the cross-section of the article to be formed in that layer. Scanning system 142 may scan the laser beam across the powder in a raster-scan fashion, or in vector fashion. Cross-sections of articles are often formed in a powder layer by scanning the laser beam in vector fashion along the outline of the cross-section in combination with a raster scan that xe2x80x9cfillsxe2x80x9d the area within the vector-drawn outline.
Referring now to FIGS. 2a through 2c, the relationship of successive fill scans among multiple parts in the same build cylinder, and among successive scanned layers, in conventional selective laser sintering processes will be described. FIG. 2a is a plan schematic view of a portion of a layer of powder at target surface 104 at which cross-sections 152a, 154a, 156a, are being formed in the current layer of powder, for three different parts or objects being fabricated in the build cycle. These cross-sections 152a, 154a, 156a are formed, in this example, by a combination of vector outlining and raster scan fills, as discussed above. As shown in the cross-sectional view of FIG. 2c, vector outline scans 160 define the outer boundaries of each of cross-sections 152a, 154a, 156a, and fill scans 162 fill the interior of each of cross-sections 152a, 154a, 156a in a raster scan manner. The vector outlines 160 are not shown in FIG. 2a (and FIG. 2b), for the sake of clarity. FIG. 2b illustrates, in plan view, the scanning of cross-sections 152b, 154b, 156b in the next layer of powder.
As shown in FIGS. 2a through 2c, the rastering of fill scans 162 are carried out in an xe2x80x9cx-fastxe2x80x9d manner, in which each scan of the laser beam is parallel to the x-axis. Conversely, the xe2x80x9cslowxe2x80x9d axis in this example is the y-axis, as the scan path is incremented in the y-direction after completion of each x-direction scan. Typically, the direction in which the scans increment alternate from layer to layer. In this example, the slow axis direction in cross-sections 152a, 154a, 156a (FIG. 2a) is the +y direction, while the slow axis direction in the next cross-sections 152b, 154b, 156b (FIG. 2b) is the xe2x88x92y direction.
The spacing between adjacent fill scans 162 is defined by a distance L between adjacent fill scans 162, as shown in FIGS. 2a and 2c. Distance L, or at least its maximum specification value, is defined according to a tradeoff between structural strength of the sintered article (which increases with decreasing L) and the speed of manufacturing (which of course increases with increasing L). It is contemplated that distance L will depend upon the particular application of the resulting article, upon the specific powder material used, and other factors.
The spacing between adjacent fill scans 162 of course only partially defines the location of scans 162; the absolute positioning of fill scans 162 within a layer also depends upon the location of the initial scan in the cross-section. According to this conventional method, the position of fill scans 162 within a given cross-section 152a, 154a, 156a is determined relative to the outer boundary of that cross-section, and depends upon distance L. This positioning is based on the outer boundary, even if this outer boundary is not vector traced by the laser beam. FIG. 2c illustrates, for cross-section 154a in which the slow axis incremental direction is the +y direction, that the first fill scan is set at a position that is distance L from the right-most vector scan 160 (measured centerline-to-centerline). Each successive fill scan 162 in cross-section 154a is then separated by distance L from the previous fill scan 162 (also measured centerline-to-centerline), continuing until the last fill scan 162 is made within cross-section 154a. In FIG. 2c, fill scans 162 are shown schematically in a non-overlapping manner for clarity; actually, adjacent fill scans 162 will overlap one another so that the powder at adjacent fill scans 162 will fuse together into a mass.
Referring back to FIG. 2a, the definition of the position of fill scans 162 within cross-sections 152a, 154a, 156a based upon the boundaries of each cross-section results in the fill scans 162 not necessarily aligning collinearly with one another. For example, fill scans 162 of cross-section 154a are offset, in the y-dimension, from fill scans 162 of cross-sections 152a, 156a. As evident from FIG. 2a, this offset among cross-sections 152a, 154a, 156a causes a large number of scan lines to be traced in the fabrication of these articles.
Referring now to FIG. 2b, according to this conventional method, cross-sections 152b, 154b, 156b are next formed, after the dispensing and spreading of the next layer of powder over that in which cross-sections 152a, 154a, 156a were formed. Cross-sections 152b, 154b, 156b are then formed by way of vector scans 160 (FIG. 2c) and fill scans 162. In this example, as shown in FIG. 2c, cross-sections 152b, 154b, 156b are identical (in the x and y dimensions) to cross-sections 152a, 154a, 156a, and as such vector scans 160 overlie one another in these two layers.
However, as noted above, the direction of slow axis incrementing is opposite for cross-sections 152b, 154b, 156b relative to cross-sections 152a, 154a, 156a. In this example, cross-sections 152b, 154b, 156b are incremented in the xe2x88x92y direction, while cross-sections 152a, 154a, 156a are incremented in the +y direction. As shown in FIG. 2c, the first fill scan 162 proceeding from the left-most vector scan 160 along the xe2x88x92y axis is separated from this vector scan 160 by distance L. Each successive fill scan 162 is then separated from the preceding fill scan 162 by distance L, as in the previous case of cross-sections 152a, 154a, 156a. 
It has also been observed, in connection with this invention, that this conventional definition of the location of fill scans 162 based upon the outer boundaries results in fill scans 162 that have no relation to one another, when considered among layers. For example, as evident from FIG. 2c, the spacing between a fill scan 162 in an upper layer and the adjacent fill scans 162 in the layer immediately below is not uniform. In the example of FIG. 2c, fill scan 162 in cross-section 154b, is separated from one adjacent fill scan 162 in cross-section 154a by a distance d1, and from the other adjacent fill scan 162 in cross-section 154a by a distance d2 that is much smaller than distance d1. The strength of bonding between cross-sections 154a, 154b is therefore limited by the larger distance d1. The worst case of this spacing will occur when fill scans 162 in adjacent layers exactly line up with one another, such that distance d2 will be at a minimum and distance d1 will be at a maximum.
Through geometric analysis, it has been observed, in connection with the invention, that the distance L between adjacent scans in the same layer defines the maximum possible distance d1 that may occur in the fabrication of a given article. Conversely, the structural strength of the sintered article, which depends in large part on the maximum distance d1 between adjacent fill scans 162 in adjacent layers, limits the spacing distance L between fill scans 162 in the same layer. In order to guarantee the desired structural strength, the spacing distance L must be selected assuming the worst case condition of fill scans 162, namely where fill scans 162 in successive layers overlie one another. However, many articles will be formed in which the worst case condition is not present, and therefore the actual distances d1 will be less than the maximum. In these cases, therefore, the spacing distance L between fill scans 162 in the same layer, defined according to the worst case condition, will be smaller than necessary, resulting in a longer build time for each cross-section of the article than is necessary to achieve proper structural strength.
By way of further background, U.S. Pat. No. 5,711,911 describes numerous techniques for ordering vector scans in the formation of an object from a liquid photopolymer by way of stereolithography. The techniques disclosed in this document address various limitations in the texture and thickness of photocured liquids. One of these disclosed techniques involves the interleaving of scans within the same layer of liquid photopolymer. Specifically, the reference discloses a layer of liquid photopolymer that is scanned, in a first pass, using non-consecutive fill scan vectors; a second pass completes the photoexposing process by scanning those scan lines between the scans of the first pass.
It is therefore an object of the present invention to provide a method of fabricating one or more articles by selective laser sintering in which the build time in each layer is minimized.
It is a further object of the present invention to provide such a method in which the structural strength of the fabricated articles is not degraded despite a reduction in the number of fill scans.
It is a further object of the present invention to provide such a method in which the structural strength of the fabricated articles is uniform.
Other objects and advantages of the present invention will be apparent to those of ordinary skill in the art having reference to the following specification together with its drawings.
The present invention may be implemented into the selective laser sintering of a three-dimensional article, in which the article is formed in layerwise fashion by the sintering, or melting and resolidification, of a powder. According to this invention, the cross-sections of the articles formed in a given layer are raster-scanned, with a selected line-to-line spacing between fill scans, and beginning from an arbitrary position in the layer. In the next layer of powder that is dispensed over the prior layer, the cross-sections of the articles are raster-scanned with the same spacing, but with the location of the scan lines substantially centered between the locations of the scan lines in the previous layers. By locating scan lines in successive layers relative to one another, rather than relative to the boundaries of the object cross-section in that layer, the number of scans required for the formation of the article or articles can be reduced, perhaps by as much as a factor of two, without degrading the structural strength of the article so formed.